Bus coupler

ABSTRACT

Bus coupler operates with a transformer and a comparator which is connected downstream in the signal-processing branch. The comparator threshold is raised as a function of the signal in the case of larger signals relative to small signals.

BACKGROUND OF THE INVENTION

The present invention relates to a bus coupler which operates with atransformer and with a comparator that is connected downstream in asignal-processing branch.

Such a bus coupler is disclosed (FR-A-2,443,770, FIG. 1). The buscoupler of the comparator operates there with a statically setthreshold. This data bus system operates with a terminating impedance atthe end of each bus line.

In the case of data bus systems, it is known to pass both data andinformation signals together with a supply voltage on a bus conductor.Such a system is disclosed in EP-A-0,365,696. In this case, the busconductor is supplied from a voltage supply having a DC voltage which isbalanced with respect to ground. A plurality of subscribers can beconnected to the bus conductor. Those subscribers can pick off both thesupply voltage and an information signal. Information signals can alsobe transmitted to the bus by the subscribers. A transformer with acapacitor is used for separating received information signals or foremitting information signals to the bus. In the case of such aninductive bus coupler, the capacitor separates the potentials of theenergy supply for the on-board network and closes the signal paththrough the transformer winding on the bus side. The system operateswithout any terminating impedance at the end of the bus line.

For transmission of bit pulses, the inductance L of the coupler shouldbe as large as possible, since the load of a transmitter which operateson the bus comprises a very large number of parallel-connected stations,each with their own bus coupler. At the same time, the total loadimpedance should be as low as possible, so that the magnetizationcurrent surge is not too large. If a comparator is used in thesignal-processing branch of a bus coupler, relatively powerful signals,such as those from transmitters operating close to the bus, should, onthe other hand, be received and even powerful interference signalsshould be attenuated or suppressed.

SUMMARY OF THE INVENTION

The present invention provides a bus coupler which attenuates relativelypowerful signals more severely than weak signals.

A bus coupler according to the present invention has a comparatorconnected downstream in the signal processing branch. The comparatorthreshold is raised as a function of the signal in the case of largesignals relative to small signals. A capacitor which is connected inseries with and upstream of the comparator input and a resistance branchwhich is in parallel with the capacitor are suitable for this purpose,for example. In the case of a positive signal which is received at thecapacitor, on its other side, a correspondingly negative signal issupplied to the comparator input, so that only a comparatively largepositive pulse could reproduce the old signal state at the output of thecomparator. In other words, the threshold value of the comparator israised, as a result of which a greater attenuation effect is achievedfor powerful pulses.

The present invention also promotes the demagnetization of thetransformer by compensating pulses, as is known per se fromEP-A-0,379,902, but in this case ensures that the demagnetization cantake place in a manner which is attenuated as little as possible. Inthis instance the bus coupler may include a resistance branch that ispartially bridged by a valve. The valve is designed to have an open pathfor a working pulse. The path is interrupted for a compensating pulse.The valve may have a response voltage such that the valve is blocked forsmall signals. Of course, the valve could be replaced with multiplevalves. In consequence, contradictory requirements are satisfied.

According to this, as initially stated, the inductance of thetransformer is kept large and, on the other hand, the resistance in theresistance branch is kept small. Furthermore, the demagnetization duringthe output signal following the information signal, in order todemagnetize the transformer, is provided by an inductance which isreduced by the capacitance, with an increased resistance in theresistance branch. This increased resistance is obtained by means of theblocked valve path with the resistance which is not yet bridged.

Nevertheless, furthermore, as already indicated, a large input signal isattenuated when viewed from the output of the comparator, so that theexcessive signals from close transmitters can be reduced to desirablelevels and interference signals, especially as a consequence ofcompensating pulses for demagnetization of the transformer, areattenuated.

The time constant τ, as a product of the resistance and capacitance, iscorrespondingly larger for the compensating pulse than for the bitsequence of an information signal, so that the compensating oscillationsare correspondingly less attenuated. In a further embodiment theresistance branch of the coupler includes at least two resistors andeach resistor is bridged by a valve having its own predeterminedthreshold in the forward direction and that threshold value increasesalong the now direction of the resistors. In the case of such a buscoupler an increasingly severe reduction is obtained for increasinglystrong signals by correspondingly raising the threshold value of thecomparator. Logarithmic characteristics can in particular thus also beachieved.

It is also possible to provide that at least two branch resistors, whereone resistor is in the range of 2:1 to 4:1. Also, the discharge timeconstant can be selected to be in the range of 10 to 20 times the timeconstant of one bit of the signal processing branch.

If either modification is selected, particularly favorable conditionsare achieved in practice.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be explained in more detail withreference to exemplary embodiments which are reproduced schematically inthe drawing:

FIG. 1 shows a bus coupler.

FIG. 2 shows a diagram of the voltage with respect to time for atransformer and operational amplifier without the measures according tothe invention, the lower curve showing the conditions after thetransformer and the upper curve the response at the output of thecomparator.

FIG. 3 shows a diagram of the voltage and of the current with respect totime for the signal-processing branch with the measures according to theinvention, the upper curve showing a voltage for driving the comparatorand the lower curve showing the current for a long information word atthe output of the comparator.

FIG. 4 shows a diagram of the voltage with respect to time in thesignal-processing branch with the comparator being controlled accordingto the present invention

FIG. 5 shows an exemplary embodiment for the circuit group, which isarranged upstream of the comparator, between said comparator and theconverter.

FIG. 6 shows a further exemplary embodiment for the circuit groupaccording to FIG. 5.

In FIG. 1, a bus coupler 3 is inductively connected to a bus 1, thatincludes the conductors 2, by means of a transformer 4. The busconductors 2 carry DC voltage for the on-board power supply from buscouplers and AC voltage for signal processing. The DC voltage for theon-board network of the bus coupler 3 is picked off via on-board networklines 5. A coupling capacitor 6 closes a through path for AC voltagesignals for signal processing, between the two primary windings of thetransformer 4. A comparator 8 is connected to the secondary winding 7 ofthe transformer 4. According to FIG. 1, a circuit arrangement 9 isarranged upstream of its input side. The output side 10 of thecomparator 8 supplies the conditioned signals of the signal-processingbranch on for further evaluation. According to the present invention,the comparator 8 is operated such that its comparator threshold israised as a function of the signal, in the case of larger signals,relative to smaller signals. A suitably dimensioned capacitor 11 in thecircuit arrangement 9 is suitable for this purpose, which circuitarrangement 9 can in general be understood as a four-pole network, thecapacitor 11 being connected in series with and upstream of thecomparator input.

In order to operate advantageously with compensating pulses fordemagnetizing the transformer, in the exemplary embodiment according toFIG. 1, a resistance branch 12 in the circuit arrangement 9 isfurthermore connected upstream, in parallel with the input of thecomparator 8. Such a resistance branch also assists the discharging ofthe capacitor 11.

The lower curve in the diagram according to FIG. 2 can be measured atthe measurement point 13, downstream of the transformer. It isdetermined that an induction voltage 18 occurs between two informationvalues 15 and 16 from sequences of individual bits 17. The inductionvoltage 18 is reduced to compensating pulses as a result of thedemagnetization processes in the transformer 4 and is amplified, inparticular by charge reversals on the coupling capacitor 6. In theabsence of the circuit arrangement 9 and without any special control ofthe comparator 8, with a voltage response according to the lower curvein FIG. 2, a voltage response according to the upper curve according toFIG. 2 is then produced at the measurement point 14 at the output of thecomparator 8, according to FIG. 1, without the circuit arrangement 9.Without special precautions, the induction voltage 18 is then amplifiedto form a false pulse 19 which is in the order of magnitude of theinformation signals or information values 20 and 21.

In the diagram according to FIG. 2, the abscissa shows time t in unitsof 0.5 ms, and the ordinate shows the voltage U in units of 5 V.

FIG. 3 shows control according to an embodiment of the present inventionin which the comparator threshold is controlled as a function of thesignal. The abscissa shows time in units of 1 ms, and the ordinate thevoltage U in units of 0.5 V. The upper curve shows the response of thevoltage 22 at the control capacitor 11. The lower curve shows thecorresponding current response in units of 5 mA. A long word having asequence of individual bits 23 is shown. The voltage 22 shows the extentof displacement of the comparator threshold. Relatively high signals arethus amplified more weakly by the comparator, and relatively weaksignals are amplified without interference.

In the diagram according to FIG. 4, the voltage U1 is shown at themeasurement point 13 of the signal-processing branch according to FIG.1, and U2 between the circuit arrangement 9, which is arranged upstreamof the comparator 8, and the comparator. The circuit arrangement 9 has acapacitor 11, which is connected in series with and upstream of theinput of the comparator 8, and a parallel resistance branch 12, theresistance branch being partially bridged by a valve 24 or by aplurality of valves. The valve has a polarity such that it is open forthe working pulse of the valve path and is interrupted for thecompensating pulse. The compensating pulses for demagnetizing theconverter coils of the transformer 4 are in this case conveyeddeliberately, but their signal effect downstream of the comparator isattenuated or switched off. Since the voltage U2 downstream of thecircuit arrangement 9 or of the four-pole network does not exceed thezero line, there is no longer any signal downstream of the comparator tothe compensating pulses 18 in the voltage U1 in the voltage U2. Beforethe induction voltage 18, the voltages U1 and U2 show a sequence ofindividual bits 23, which are part of a complete information word.

The circuit arrangement 9 according to FIG. 1 is designed in theexemplary embodiment shown such that the resistance branch 12 is bridgedwith respect to R1 by a valve 24 of such polarity that the valve path isopen for the working pulse and is interrupted for the compensatingpulse. For the working pulse, the time constant τ is then the product ofthe capacitance of the control capacitor 11 and the resistor R2. For thecompensating pulse, the time constant τ is then the product of thecapacitance of the control capacitor C1 and the sum of the resistors R1and R2. The large time constant for the compensating pulse correspondsto low attenuation, and the small time constant for the operating pulseto a correspondingly large attenuation. The compensating pulses are thusgiven a long time duration, but their influence on signal formationdownstream of the comparator is suppressed or switched off. If the valve24 has a response voltage such that the valve is blocked for signalswhich are defined as small, such small signals are not attenuated much.

In the exemplary embodiment of the circuit arrangement 9 according toFIG. 5, the control capacitor 11 is provided with a limiting diode 25 inthe path of the working pulse, which limiting diode 25 limits thecontrol of the regulation shift of the comparator 8 in a defined manner.In the exemplary embodiment according to FIG. 5, the resistor R1 isbridged by two valves 24, irrespective of the limiting diode, so that,in the case of valves which are identical to that in FIG. 1, double theresponse voltage is achieved is reached. The resistor R1 is then notbridged until this higher voltage is reached. In the case of theexemplary embodiment according to FIG. 6, the resistors R1.1, R1.2 andR1.3 are bridged by an increasing number of valves 24. R1.1 by a valve24, R1.2 by two valves 24 and R1.3 by three valves 24. Correspondingcomponents with the correspondingly higher response voltage can, ofcourse, also be used instead of a plurality of valves. If two or moreresistors are bridged, said resistors are then in each case bridged byvalves having a predetermined threshold voltage in the forwarddirection, which voltage rises in the ascending row direction, so thatlogarithmic or other desired regulation curves can be obtained.

For a simple exemplary embodiment of the circuit arrangement 9 accordingto FIG. 1, advantageous value ranges for optimizing the conveyance ofcompensating processes for demagnetization and for suppression ofinterference signals and of neighboring powerful transmitting signals,the following value ranges are favorable:

the ratio of the resistors R2:R1 may be in the range from 1:2 to 1:4.

The discharge time constant C11×(R1+R2) may be on the order of 10 to 20times the time constant of one bit of the signal-processing branch.

I claim:
 1. A bus coupler comprising:an input circuit with an input forcoupling to a signal-carrying bus; and a comparator with inputs coupledacross an output of the input circuit, wherein:the input circuitattenuates a signal at its input as a function of the signal's amplitudeso that the attenuation increases as the signal's amplitude increases;and the input circuit includes:a resistance branch coupled across theinputs of the comparator, the resistance branch being partially bridgedby a valve, the valve conducting for an input signal with an amplitudeat least as large as a predetermined threshold and the valve beingblocked for an input signal with an amplitude less than thepredetermined threshold, and a control capacitor coupled between theinput of the input circuit and one of the inputs of the comparator. 2.The bus coupler of claim 1, wherein:the valve conducts when a signalpulse is applied to the bus coupler and is blocked when a compensatingpulse is applied; and the signal pulses are used to convey informationand the compensating pulses are used to demagnetize a transformer bywhich the bus coupler is coupled to the bus.
 3. The bus coupler of claim2, wherein the resistance branch includes at least two resistors thatare each bridged by valves having a predetermined threshold voltage inthe conducting direction, the valves having progressively increasingthreshold voltages.
 4. The bus coupler of claim 2, wherein theresistance branch includes a bridged resistor and an unbridged resistor,the resistance of the bridged resistor being two to four times theresistance of the unbridged resistor.
 5. The bus coupler of claim 3,wherein a time constant of a discharge of the control capacitor throughthe resistors of the resistance branch is on an order of magnitude of 10to 20 times a period of one bit of the signal pulses.
 6. The bus couplerof claim 1, wherein the control capacitor is bridged by a limitingdiode.